The present invention relates generally to lithographic devices for the fabrication of integrated circuits on semiconductor wafers. More specifically, the present invention relates to a low stiffness, suspension system and method for suspending a stage of the lithographic device.
Lithographic devices are commonly used to transfer an image from a recticle onto a semiconductor wafer. A typical lithographic device includes a support frame, an illumination source, a reticle stage, a reticle stage base, a wafer stage, a wafer stage base, a metrology system, and a lens assembly that cooperate to transfer an image of an integrated circuit from the reticle onto the wafer. The reticle is supported and moved by the reticle stage relative to the reticle stage base and the wafer is supported and moved by the wafer stage relative to the wafer stage base.
The integrated circuits transferred onto the wafer are extremely small. Accordingly, the relative alignment between the wafer and the reticle is critical to precisely transferring the image onto the wafer and to creating high density semiconductor wafers. In order to obtain precise relative alignment, the position of the reticle and the wafer are constantly monitored by the metrology system. Subsequently, with information from the metrology system, the reticle stage moves the reticle relative to the reticle stage base and the wafer stage moves the wafer relative to the wafer stage base to precisely adjust the alignment between the reticle and the wafer.
Typically, one or more reticle stage movers are used to precisely move the reticle stage relative to the reticle stage base. Similarly, one or more wafer stage movers are used to precisely move the wafer stage relative to the wafer stage base. Each stage is typically supported above its respective stage base with air bearings, mechanical guides or magnetic bearings.
Unfortunately, existing ways of supporting the stage above the stage base are not entirely satisfactory. For example, air bearings typically permit substantially frictionless planar motion along an X axis, along a Y axis and rotation around a Z axis. However, air bearings in precision machines traditionally use a relatively small air gap and have a relatively high stiffness. This arrangement does not permit significant motion along the Z axis or rotation about the X axis and the Y axis. Further, the air bearings transmit vibration and shock forces along the Z axis to the stage. The vibration and shock forces reduce the accuracy of the device and lower the quality of the integrated circuits formed on the wafer. Alternately, low-stiffness air bearings are not stable and have turbulent flow problems.
Mechanical guides are not completely satisfactory because of friction during movement and limited degrees of freedom of motion. Magnetic bearings are not completely satisfactory because they typically require a large target of magnetic material for the levitation of the magnets. In many situations, including lithographic devices, this approach is problematic because the target interferes with the other components of the lithographic device. Further, magnetic bearings consume a large amount of power and generate a significant amount of heat.
In light of the above, it is an object of the present invention to provide a suspension system and method for suspending a stage spaced apart from a stage base. Yet another object of the present invention is to provide a suspension system for a stage that allows for large displacement, frictionless motion along the X axis, along the Y axis and around the Z axis and small-displacement, low-stiffness motion along the Z axis, and rotation about the X axis and the Y axis. Another object of the present invention is to provide a low-stiffness suspension system for a stage that allows for adjustments along the Z axis with minimal restrictions. Still another object of the present invention is to provide a frictionless suspension system that permits six degrees of freedom motion.
The present invention is directed to a suspension system for maintaining a stage spaced apart from a stage base which satisfies these needs. The suspension system includes one or more spaced apart, suspension assemblies. Each suspension assembly includes a fluid pad and an adaptable section. A fluid is directed from the fluid pad towards the stage base to create a fluid bearing between the fluid pad and the stage base. The fluid bearing allows for large displacement, frictionless motion of the suspension assembly and the stage along an X axis, along a Y axis and rotation about a Z axis. The adaptable section secures the fluid pad to the stage. The adaptable section allows for relative movement between the fluid pad and the stage. More specifically, the adaptable section allows for small displacement, low-stiffness motion between the fluid pad and the stage along the Z axis, and rotation about the X axis and the Y axis.
Basically, each suspension assembly utilizes the relatively soft, adaptable section in series with a relatively high stiffness, fluid bearing. With this design, the suspension assembly permits six degrees of freedom motion and provides some vibration isolation along the Z axis. Suitable adaptable sections include a flexible, fluid chamber, a coil spring, a constant force spring, a leaf spring, a flexible member, and/or an air spring. As provided herein, the stiffness of the adaptable section can be varied. For example, a controller can control the stiffness of the adaptable section by changing the characteristics of the chamber fluid in the fluid chamber.
Additionally, each suspension assembly can include a restrictor which inhibits motion of the fluid pad transverse to a suspension axis of each suspension assembly. The restrictor maintains the fluid pad substantially parallel with the stage base. As provided herein, the restrictor can encircle a portion of the suspension assembly and inhibit motion of the fluid pad transverse to the suspension axis. Alternately, the restrictor can extend between the fluid pad and stage and inhibit motion of the fluid pad transverse to the suspension axis.
One or more movers can be used to move the suspended stage relative to the stage base. Further, the movers can be used to move the stage relative to the fluid pads of each suspension assembly. Suitable movers include one or more linear motors, planar motors, voice coil actuators and/or El core motors.
The present invention is also a method for suspending a stage spaced apart from a stage base. The method includes creating a fluid bearing between a fluid pad and the stage base and securing the fluid pad to the stage with an adaptable section.
Importantly, each suspension assembly permits six degrees of freedom motion between the stage and the stage base, provides some vibration isolation along the Z axis and supports the stage without electrical power consumption of the movers.